The present invention relates to a semiconductor integrated circuit device and, particularly, to the technique applicable effectively to the input protective element of semiconductor integrated circuit devices (bipolar-CMOS LSIs) incorporating bipolar transistors and MISFETs.
Lately, research and development has been rather active in bipolar-CMOS LSIs in which bipolar transistors and complementary MISFETs are formed on the same substrate, as described in publication IEDM 1985, Technical Digest, pp. 423-426, for example.
Japanese Patent Application No. 61-65730, which was filed by the applicant of the present invention, deals with an input protective element for bipolar-CMOS LSIs. In this protective element, n-channel MISFETs, with their source and gate electrodes being connected to the ground voltage, are used for voltage clamping. The n-channel MISFETs are formed in a p-well which is provided on a p-type silicon substrate through a p.sup.+ type isolation diffused region (P-iso). The input protective element is designed for the following. If an abnormal positive voltage (e.g., 15 volts) higher than the drain-source breakdown voltage BV.sub.DS is applied to an input terminal, the current is led to the ground mainly on the basis of the drain-source punch-through phenomenon. If an abnormal negative voltage in excess of the forward build-up voltage V.sub.F of a parasitic diode made up of the drain region of n-channel MISFET and the p-well is applied to an input terminal, the current is led through the parasitic diode to the input terminal so as to prevent the abnormal voltage application to the internal gate circuit (input buffer). The n-channel MISFET and parasitic diode in unison serve to protect the internal gate circuit from electrostatic destruction. With the intention of limiting the substrate current when a negative voltage attributable to the undershooting of input signal is applied, the input protective element is provided with a polycrystalline silicon resistor between the drain of the n-channel MISFET and the input pad.